To avoid the side-effects of drugs on the brain, analgesic or anesthetic drugs can be delivered to the spinal cord by placing the drugs outside of the membranous sac containing the spinal cord. Between this sac, called the dura, and the overlying spinal ligaments, is a potential space called the spinal epidural space (SES). It is a potential space because normally the anatomy here is juxtaposed until the space is crested. Placing drugs in the SES blocks spinal cord functions including pain transmission permitting either pain control (analgesia) or complete loss of all sensation (anesthesia) for surgery.
In clinical practice, locating the SES with a needle is technically difficult. The greatest danger for the novice is to sense the change in resistance as the needle passes through the spinal ligaments before the needle inadvertently passes through the SES and penetrates the dura. In other words, one seeks to reach the SES and stop before going through the dura.
Epidural anesthesia or analgesia is one of the most popular regional anesthetic procedures employed for surgery, obstetrics, postoperative analgesia, and chronic back pain management. The potential risk involved in this procedure is the accidental puncture of the dura. Identification of the precise moment when the needle is advanced into the epidural space decreases the likelihood of that risk.
Present methods for identifying this space fall into two categories: the "loss of resistance" and the "hanging drop" techniques. The former is the most commonly adopted technique to identify the space due to the lack of precision in the latter.
The loss of resistance technique involves direction of the epidural needle through the skin into the interspinous ligament. Then, the stylet of the needle is removed and an air-tight and free sliding glass syringe, containing air, or saline is connected to the needle. If the needle tip is properly positioned within the substance of the interspinous ligament, injection will not be possible; this is defined as the feeling of resistance. At this point, most textbooks suggest for the noninjecting hand to advance the needle with the thumb and index finger grasping the hub of the needle while the dorsum of the hand rests on the patient's back for stabilization. The injecting hand is placed on the plunger of the syringe with gentle but continuous pressure. As the needle passes through the ligarnentum flavum and enters the epidural space, a sudden loss of resistance occurs. The medication can then be injected with precision into the epidural space.
There are several disadvantages to this technique. First, the method described above is especially difficult for a novice because experience is required to obtain coordination of the two hands which are functioning differently. Next, because of the lack of an objective visual indicator, this method is difficult to supervise and results in a high incidence of dural puncture among novices.
The "loss of resistance" technique has widely been alternated involving a two-handed grip on the syringe and needle with continuous firm pressure on the hub. As the needle is advanced a few millimeters, one will stop and check the location of the needle by gently depressing the plunger and confirming whether the needle tip is still within the ligament or has moved to the area where loss of resistance occurs. The apparent disadvantage of this method is that in between stops, the needle could have advanced through the epidural space and punctured the dura.
The "hanging drop" technique capitalizes upon the loss of pressure experienced when the needle enters the epidural space. A drop of saline solution is placed on the open hub of the needle. The drop "hangs" on the needle until the needle enters the epidural space, when the needle tip indents the dura resulting in negative pressure and the drop is "sucked" into the needle from the change in pressure. This indicates that the needle should be stopped as it has entered the epidural space.
Regardless of the technique used, locating the epidural space can be a difficult endeavor for both novices and experts because it is a potential space between two tissues held together by a slight negative pressure. Dural puncture is the greatest risk when there is error and sequelae of this mistake can range from spinal headaches to lethal total spinal anesthesia.